1. Field of the Invention
The present invention relates to laser beam profilers in general and in particular to a laser beam profiler which comprises a multimode diode laser optical interferometer that avoids problems caused by the use of a single mode diode laser where the laser will mode-hop and cause errors in the accuracy of the interferometer.
2. Description of the Prior Art
A laser beam profiler is an analytical instrument which is used for measuring the profile, i.e. dimensions, of a laser beam along one or more axes. In practice, a photodetector is used to detect the radiation from the laser as a knife-edge, slit, pinhole, or the like, is moved through the beam. In a preferred embodiment a knife-edge is used for measuring a small laser beam. The position of the knife-edge in the beam can be measured using an interferometer. The output of the photodetector and the interferometer are used in conjunction to provide an output corresponding to the measured laser beam profile.
Optical interferometers have been used in laboratory and industrial applications for many years. Interferometers are generally used where the amount of movement required is not large but the precision required for measuring the movement is great.
With the advent of the laser, interferometers became useful in a wide variety of applications including machine control and particularly in the semiconductor industry. The development of the diode laser, particularly the single mode diode laser which has a relatively long coherence length, further provided a laser source that is very attractive because of its low cost, compact size and low power requirements, thus making it useful in such devices as laser beam profilers.
While the single mode diode laser is attractive, it also has problems associated with it. One of the most difficult problems is the laser's tendency to mode-hop in the presence of power changes, temperature changes and possibly with natural aging of the device. Mode-hopping has also been observed in response to changes in the percentage of the laser's output radiation that is reflected back into the laser.
Various methods have been used in an attempt to stabilize the laser so that it does not mode-hop. For example, temperature control has been used in conjunction with methods to control the output power of the laser. The output power of the laser normally is controlled by use of a photodiode that monitors the output power and is connected, in conjunction with external electronics, to feed back a signal that restricts the output power to a predetermined level. Such a method used to control mode-hopping is taught in U.S. Pat. No. 4,817,098 to Horikawa entitled "Semiconductor Laser Divider System".
Even by employing means to control temperature and power, it cannot be assumed that the laser mode-hopping transition points will remain constant over the life of the device; and, therefore, it cannot be assumed that an interferometer will not have, some time in the future, an unstable point move into the region that has been preselected at an earlier point in time as being a stable point of operation. Tests have been performed where diode lasers have been aged for 2000 hours at 20 .degree. C. and have experienced changes in mode-hopping transition points.
Methods have been employed to produce an operating system that will function in the presence of mode-hopping where the power of the laser is toggled between a plurality of power levels in an attempt to find a point where mode-hopping does not occur and where an accurate measurement can be made. This method is useful when the designer is forced to avoid the added cost of temperature stabilization of the laser. In using this method the designer has to assume that, over the normal operating environments, by varying laser power, the laser will move into a temperature/power regime where mode-hopping will not occur. However, such a multiple-power level technique is undesirable for reasons in addition to its added complexity. Experience has shown that there are some single mode lasers that will mode-hop over a wide variation of power levels. Also, an interferometer that is designed to avoid mode-hopping by changing power levels has the additional problem of having to be designed to be accurate over a wide range of signal levels in its detection circuitry. Such a circuit would not be as accurate as a circuit that was controlled in power and optimized to be accurate at one power level. Finally, a circuit that requires the output to be changed to many various power levels in an attempt to find a stable power level also would require more time to make a measurement.
The problem of mode-hopping is twofold. One problem is that the actual change in frequency of the light causes the distance between interference fringes to change and, therefore, the accuracy of the measurement made with the interferometer to degrade. The second problem is that mode-hopping generally entails a change in the optical output power (a mode-hopping noise signal) that changes the accuracy of the interferometer and decreases its effectiveness.
Several other patents have issued which deal with the problem of mode-hopping, including U.S. Pat. No. 4,807,992 to Noguchi et al entitled "Method of Detecting Semiconductor Laser Mode-Hopping and Semiconductor Laser Beam Source Apparatus"; U.S. Pat. No. 4,733,253 to Daniele entitled "Apparatus and Method for Detecting and Eliminating Diode Laser Mode-Hopping"; and U.S. Pat. No. 4,737,798 to Lonis et al entitled "Laser Diode Mode-Hopping Sensing and Control System".